Rotationally Resolved Electronic Spectroscopy of UO₂

Rotationally resolved electronic spectra have been recorded for gas-phase UO₂. Analysis of the rotational fine structure confirms that the ground state is X³Φ_2u, derived from the O^2-U⁴⁺(5f7s)O^2- electronic configuration. All transitions observed below 14500 cm^-1 were assigned to the [11.51]3_g ↔ X³Φ_2u transition, which correlates with the metal-centered 5f7p ↔ 5f7s electron promotion. The symmetric stretch and bending vibrational modes were active in the spectrum. Ground-state vibrational fundamentals of 853(5) and 133(5) cm^-1 were determined along with a vibrationally averaged bond length of 1.781 Å. Spectra for the [17.68]4_g ↔ X³Φ_3u transition exhibited the same lower state vibrational frequencies, confirming that the X³Φ_3u state also arises from the 5f7s configuration. Fluorescence decay lifetimes for the excited states were in the range of 1.5-4.7 μs, values that were anomalously long for electric dipole-allowed transitions. The long lifetimes are attributed to extensive mixing with a dense background of vibronic dark states.